The present invention relates to electrostatographic image development using magnetic development rollers, and more specifically to a molding assembly for producing electrostatographic magnetic development rollers having precise magnetic development fields.
In the well-known process of electrostatographic printing, a charge retentive surface, typically known as a photoreceptor, is electrostatically charged, and then exposed to a light pattern of an original image to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharged areas on the photoreceptor form an electrostatic charge pattern, known as a latent image, conforming to the original image. The latent image is developed by contacting it with a fine electrostatically attractable powder known as "toner." Toner is held on the image areas by the electrostatic charge on the photoreceptor surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate or support member (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface.
The process as described above is useful for light lens copying from an original, or for printing electronically generated or stored originals such as with a raster output scanner (ROS), where a charged surface may be imagewise discharged in a variety of ways.
In the process of electrostatographic printing, the step of conveying toner to the latent image on the photoreceptor is known as "development." The object of effective development of a latent image on the photoreceptor is to convey changed toner particles to the latent image at a controlled rate so that the toner particles effectively adhere electrostatically to the image areas on the latent image.
A commonly used technique for development involves the use of a two-component developer material, which comprises, in addition to the toner particles, a quantity of magnetic development carrier granules or beads. The toner particles adhere triboelectrically to the relatively large carrier beads, which are typically made of steel. When the two-component developer material is placed in a magnetic development field, the carrier beads with the toner particles adhering thereto form what is known as a magnetic development brush, wherein the carrier beads form relatively long chains which resemble the fibers of a brush on a developer or development roller. The development roller is typically in the form of a cylindrical sleeve rotating around a fixed assembly of permanent magnets called a magnetic development roller. When the magnetic development brush is introduced into a development zone adjacent the photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be pulled off the carrier beads in the chain and onto the latent image.
Another known development technique involves a single-component developer, that is, a developer which consists entirely of toner. In a common type of single-component system, each toner particle has both an electrostatic charge (to enable the particles to adhere to the photoreceptor) and magnetic development properties (to allow the particles to be magnetic developmentally conveyed to the photoreceptor). Instead of using magnetic development carrier beads to form a magnetic development brush, the magnetized toner particles are caused to adhere directly to a magnetic development roller. In the development zone adjacent the photoreceptor, the electrostatic charge on the photoreceptor will cause the toner particles to be attracted from the magnetic development roller to the photoreceptor. Conventionally, magnetic development rollers each typically includes a plurality of poles for attracting the toner particles. These poles may be positioned on the periphery of the magnetic development roller at such positions to obtain optimum transfer of the toner particles to the photoconductive surface of the drum.
Magnetic development rollers have typically been manufactured with a core or body and magnets positioned on the periphery of the core. Typically the magnets are glued to the periphery of the core. The gluing of magnets to a core contributes to a series of problems. The gluing leads to positioning errors both radially and tangentially, reducing the quality of the roll. Further, added cost may be required to perform subsequent machining of the periphery of the roll to obtain needed accurate tolerances.
More recently, it has been known to mold magnetizable material about a shaft to form a magnetic development roller. The magnetizable material may be one of any suitable moldable materials but, preferably the materials include ferrite, or neodymium iron boron powder. Permanent magnets are imbedded in the molding assembly and are positioned near the periphery of the mold cavity of the assembly where the molded material is to be placed, in an attempt to transfer precise magnetic development fields from the permanent magnets to the magnetizable material within the mold cavity.
The following disclosures may be relevant to various aspects of the present invention. U.S. Pat. No. 5,181,971 discloses a method of manufacturing a magnetic development roller. The method includes the step of disposing a plurality of pairs of magnetic development poles each having the starting magnetic development pole and terminal magnetic development pole of a magnetic development line of force on the peripheral surface of a cavity in a metal mold in which a resin magnet is molded. The lines of flux are in parallel lines
U.S. Pat. No. 5,019,796 discloses an improved bar magnet and method of construction and an improved magnetic development core. An assembly of magnet is shown for use in a processing station of a printing machine. The bar magnet is formed of permanent magnet material having magnetic development domains therein that are magnetized along epicyclical curve segments. The external magnetic development flux density is improved over that of a conventionally magnetized magnet.
U.S. Pat. No. 4,557,582 discloses a magnet roll including magnet pieces adhesively secured to a supporting shaft to increase the magnetic development flux density of a pole. The pieces are disposed so that they have repelling magnetic development forces in the interface between the piece behind the pole and the piece adjacent thereto.
The use of magnetizable material molded about a shaft however is plagued with several problems. Wear of the mold during the molding process, which is attributable to the abrasive nature of the ferrite material within the mold cavity magnets relative to the mold cavity, and which can change the radial positioning of the permanent magnets, thus affecting the strength of the magnetic development fields of a molded roller. Such wear is critical because the effective field of the permanent magnet is reduced as a cubic function of the distance between the permanent magnet and the molded material in the cavity, and because the portion of the mold between the molded material and the magnet must be kept to a minimum.
Further, the circumferential positions of the permanent magnets are critical because those of the poles within the molded magnetic development roller are critical for the proper transfer of the toner within the development unit of the printing machine. The circumferential positioning of the magnetic development fields of the permanent magnets can cause the fields to interact with each other within the mold, thus making it very difficult to predict where to place the permanent magnets within the mold around the mold cavity in order to obtain a magnetic development roller with poles positioned precisely in particular locations about its circumference.
Further, manufacturers may manufacture several different such rolls, rolls with poles in various positions, yet all these rolls must have the same length and diameter in order to fit a particular machine. Such differences in the positioning of the poles requires a separate expensive mold for each particular pole configuration, thus resulting in change-over costs when manufacturing rolls with identical diameters and different pole positions.
Attempts to position the permanent magnets precisely for desired radial and circumferential induced pole positioning, usually require that the positions of the permanent magnets be determined for example by using computer modeling. However, it has been found that even computer modeling is accurate only to approximately plus or minus one to three degrees. Therefore, further trial and error costly, as well as imprecise adjustments are often necessary following even such computer modeling.
There is therefore a need for an efficient, low cost molding assembly for producing electrostatographic magnetic development rollers that have precise magnetic development fields.